1. Field of the Invention
The present invention relates generally to Coriolis type mass flow meters, and more particularly, to an improvement in the sensor thereof for minimizing the vibrational coupling between drive mode vibration and other mode vibrations thereby producing a more stable sensing device.
The present application is related to the copending applications of Andrew K. Levien, entitled Sensor Apparatus For Mass Flow Rate Measurement System, Ser. No. 822,123 filed Jan. 24, 1986, now abandoned; of Erik B. Dahlin entitled "Apparatus for Mass Flow Rate and Density Measurement", Ser. No. 775,739, filed Sept. 13, 1985, now U.S. Pat. No. 4,711,132; and of Erik B. Dahlin et al. entitled "Improved Apparatus for Mass Flow Rate and Density Measurement", Ser. No. 777,707, filed Sept. 13, 1985, now U.S. Pat. No. 4,660,421, all of which are assigned to the assignee of the present application.
2. Discussion of the Prior Art
The present invention is an improvement on multiple tube designs of Coriolis mass flow meters of the type disclosed in the above-mentioned copending applications as well those disclosed in the prior art U.S. Pat. Nos. to Cox et al, 4,127,028; Smith 4,422,338; and Smith et al, 4,491,025. The sensors used in these types of meters typically consist of oppositely driven tubular elements of looped or U-tube configuration connected by stiff isolation plates. With respect to relative motion between the tubes, these plates isolate the weld points of the tubes from the motion of the tubes and define vibrational node points in the system. In such apparatus the tube elements are vibrated in the drive mode at one frequency and are consequently subjected to Coriolis force induced vibration when mass is flowing through the tube.
The stability and accuracy of these devices are dependent upon the frequency relationships among the various natural modes of vibration of the structure. The resonant frequency of each particular mode of vibration is dependent upon the stiffness of the structure and the mass inertial properties of the structure in that mode.
The stiffness of the structure is directly proportional to the stiffness of the tube material used and also depends upon the geometry of the loop and the isolation plate location.
In the loop or U-tube type structure, drive oscillation is provided at corresponding drive points on each loop (or U-tube) and causes motion of the tubes in opposite directions. This particular mode of oscillation is normally referred to as the "drive mode".
When mass is flowing through the tubes and the drive motion is applied, Coriolis forces will be induced within the tubes and a Coriolis induced motion will be super-imposed upon the drive motion.
In addition to the drive mode oscillation and the Coriolis mode oscillation, dual loop structures are also subject to oscillation in a "roll mode" wherein the loops move in the directions tending to wind and unwind the loops. It will be appreciated that the three motions all combine to cause the sensor to experience a compound vibrational motion which will favor one or more of the modes depending upon the stiffness of the structure in each mode.
In previous designs, rigid end isolation plates have been utilized to limit the vibrational characteristics of the device to the inherent structural characteristics of the U-tubes or loops disposed between the separated end isolation plates. These characteristics will of course also be influenced by any mass attached to the U-tubes or loops as it will modify the mass inertia of the structure.
It has been recognized that one can improve the operational characteristics of a particular sensor configuration by preferentially modifying the stiffness of the structure in the various modes in order to "decouple" the vibrations, i.e., increase the separation between their natural frequencies and thus render the structure more sensitive to certain modal vibrations and less sensitive to others. One teaching of a method in which the relative characteristics of a pair of sensor loops may be selectively modified is disclosed in the above-identified copending application of Andrew K. Levien. The present invention relates to another method for preferentially modifying the stiffness of a particular sensor structure.